1. Field of the Invention
The present invention relates to an airbag which includes an outer circumferential wall comprised of flexible sheet material, and is housed in a housing in a folded-up state and inflatable with inflation gas for emergence from the housing and deployment.
2. Description of Related Art
JP 7-205738 A discloses an airbag that includes on the outer circumferential wall a vent hole and a temporary joint which is formed by tucking a part of the outer circumferential wall encompassing the vent hole and sewing the tucked region with a sewing thread in such a manner as to close off the vent hole. With such a temporary joint, the airbag inflates with a suppressed volume in an initial stage of inflation and when the temporary joint is broken due to increase of inner pressure of the airbag, the airbag is allowed to inflate in a full volume.
However, since the tucked region in the conventional airbag is formed to protrude outwardly from the airbag, when the airbag is inflated with a suppressed volume while the temporary joint is kept jointed, the tucked region protrudes partially on the outer surface of the airbag, thereby making the contour of the airbag of suppressed volume irregular. Moreover, since the tucked region is likely to be caught by a housing of the airbag or a surrounding member, the airbag sometimes fails to inflate smoothly. Further, the temporary joint cannot be located on such a location as to contact with an occupant upon deployment since it is formed in a projecting fashion on the outer surface of the airbag.
Moreover, the temporary joint of the conventional airbag is formed into a generally V shape such that rupture of the temporary joint starts from the angular portion when an inner pressure of the airbag is increased. However, with this configuration, rupture or raveling of the temporary joint propagates to two directions since the temporary joint is bifurcate starting the angular portion, and therefore, raveling or rupture of the sewing thread occurs on more than one region proximate to each other. Accordingly, even more rapid disjointing is desired.